The super Keyword

Part of the Object Oriented Programming section of Coddy's Dart journey — lesson 37 of 110.

The super keyword gives a child class direct access to its parent class. You've already seen it in constructors with super(name), but it's useful in other contexts too.

When you need to call the parent's constructor, super passes arguments up the inheritance chain:

class Vehicle {
  String brand;
  int year;
  
  Vehicle(this.brand, this.year);
}

class Car extends Vehicle {
  int doors;
  
  Car(String brand, int year, this.doors) : super(brand, year);
}

The super keyword also lets you access parent methods and properties from within the child class. This is especially useful when you want to use the parent's behavior alongside your own:

class Animal {
  String name;
  
  Animal(this.name);
  
  void makeSound() {
    print('$name makes a sound');
  }
}

class Cat extends Animal {
  Cat(String name) : super(name);
  
  void makeSound() {
    super.makeSound();  // Call parent's method first
    print('$name says meow!');
  }
}

When you call super.makeSound(), you're explicitly invoking the parent's version of the method. Without super, calling makeSound() would refer to the current class's method, potentially causing infinite recursion.

Think of super as saying "use my parent's version" - whether that's a constructor, method, or property.

Challenge

Easy

Let's build a messaging system that demonstrates how the super keyword lets child classes access and extend their parent's behavior. You'll create a hierarchy where different message types build upon a base message class.

You'll organize your code into two files:

message.dart: Define your message classes here:
- A Message class (the parent) with a String sender and String content. Include a constructor that takes both values and a method display() that prints From [sender]: [content]
- An UrgentMessage class that extends Message. It should have an additional String priority property. The constructor takes sender, content, and priority, passing the first two to the parent using super. Override the display() method to first call the parent's version using super.display(), then print Priority: [priority] on the next line
main.dart: Import your message file and demonstrate how super works in both constructors and methods:
- Create a regular Message from 'Alice' with content 'Hello there!'
- Call its display() method
- Print an empty line for separation
- Create an UrgentMessage from 'Bob' with content 'Server is down!' and priority 'HIGH'
- Call its display() method

Notice how UrgentMessage uses super in two ways: in the constructor to pass data to the parent, and in the overridden method to include the parent's output before adding its own.

Expected output:

From Alice: Hello there!

From Bob: Server is down!
Priority: HIGH

Cheat sheet

The super keyword provides access to a parent class from within a child class.

Use super() in a child class constructor to pass arguments to the parent constructor:

class Vehicle {
  String brand;
  int year;
  
  Vehicle(this.brand, this.year);
}

class Car extends Vehicle {
  int doors;
  
  Car(String brand, int year, this.doors) : super(brand, year);
}

Use super to call parent methods from within overridden methods:

class Animal {
  String name;
  
  Animal(this.name);
  
  void makeSound() {
    print('$name makes a sound');
  }
}

class Cat extends Animal {
  Cat(String name) : super(name);
  
  void makeSound() {
    super.makeSound();  // Call parent's method first
    print('$name says meow!');
  }
}

Without super, calling a method name directly refers to the current class's version, which can cause infinite recursion in overridden methods.

Try it yourself

import 'message.dart';

void main() {
  // TODO: Create a regular Message from 'Alice' with content 'Hello there!'
  
  // TODO: Call the display() method on the message
  
  // TODO: Print an empty line for separation
  
  // TODO: Create an UrgentMessage from 'Bob' with content 'Server is down!' and priority 'HIGH'
  
  // TODO: Call the display() method on the urgent message
}